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IAT sensor problem

Hi all,

I have been trying to deal with a rough idle and its driving me nuts. So i decided to deal with minor issues that may be causing it.

P1192: IAT sensor voltage low.

I backprobed the sensor and the voltage was always less than 0.5V, so the PCM isn't lying. I took the sensor and checked the resistance, started at about 7.7k ohms, i then used a hair drier to warm it up and it kept dropping to 4.2k ohms which is when i stopped the hair drier.

I then went to check the connector and it looked fine, i then followed the wire to the next intersection of the wiring harness, and it was fine.

I checked the voltage between sensor ground and battery positive, and saw 12V, meaning good ground. I then connected the multimeter positive to sensor positive and multimeter negative to battery negative, and saw the same stupidly low voltage as before.

You should put that info in your signature, so no one has to search your other posts for what kind of van you have

Assuming this is on 2002, Check the voltage on PCM Connector 1 (black) pin 37 (DB/LG) - start with unplugged sensor.
If you find low voltage, shut the van off and pull the pcm connector off - check resistance between this wire (sensor supply) and ground (it should be infinitely high = open circuit with sensor unplugged)

If everything is good in the wiring and pcm output is low, the problem is internal to pcm

At pin 37, i have the same very low voltage. And my PCM connector 1 is grey, both of the connectors are for that matter.

Checking resistance with connector unplugged and sensor unplugged, i get infinitely high resistance, but why would i get a resistance in the first place? the wires aren't connected!!!

I also checked the resistance between PCM pin 37 and the sensor source, i get over 1k ohms. For just a wire, i know thats too much, but since the sensor itself is part of the circuit (and other sensors) should i even consider this number?

At pin 37, i have the same very low voltage. And my PCM connector 1 is grey, both of the connectors are for that matter.

The connector cover and outer shell can be any color, it's usually the insert (pad which supports the pins) that is color coded. (but mistakes can be made or could be substitute parts/repair ect..)

Checking resistance with connector unplugged and sensor unplugged, i get infinitely high resistance, but why would i get a resistance in the first place? the wires aren't connected!!!

Good, the unplugged wire should be fully insulated from ground. (that's what you're testing - that the wire is not shorted to ground and thus is not pulling down the sensor supply voltage)

I also checked the resistance between PCM pin 37 and the sensor source, i get over 1k ohms. For just a wire, i know thats too much, but since the sensor itself is part of the circuit (and other sensors) should i even consider this number?

If you're probing the same wire between PCM and IAT connectors, it should only have a few ohm resistance (1-3 ohm would be ok). When both connectors are unplugged, the wire is not part of any circuit - just a simple conductor..
Assuming your reading is correct (it wasn't just poor contact of a meter probe or wrong scale [ohm vs. Kohm] you want to investigate the wiring harness and also might want to check resistance between this wire and +12v at the battery (to make sure there has no short to +12, which could affect the PCM). However it does not change the fact that PCM is not outputting the correct supply voltage (IIRC, it uses approx 5vdc for supply)

If you confirm the 1Kohm reading, you can leave the meter connected and carefully shift/twist/pull on parts of the wiring harness while checking the readout to see if you can isolate part of the wiring harness.

The two known wiring trouble spots on early 4th gen are:
Fuel injector harness melting/wearing on top of the motor (IIRC, in the vicinity of coil & ps reservoir)
and under the battery tray if acid (h2so4) makes its way down to the wiring bundle

both have been discussed in detail (and with pics), a little searching should turn it up..

If that won't work, would it be okay to put my own 5V supply to the source of the wire? Since the current supply is only 0.2V... The IAT sensor simply alters that resistance.

One thing i would like to mention is that I used a hair drier to test the sensor and monitored the resistance across the pins on the sensor. As temp increased, resistance decreased, isn't it suppose to be the other way??

If that won't work, would it be okay to put my own 5V supply to the source of the wire? Since the current supply is only 0.2V... The IAT sensor simply alters that resistance.

That won't help you any - the 5v sensor supply is the 'sense' side of the IAT, the ground is just 'sensor ground' (ground point at the PCM for accuracy)

One thing i would like to mention is that I used a hair drier to test the sensor and monitored the resistance across the pins on the sensor. As temp increased, resistance decreased, isn't it suppose to be the other way??

The Following User Says Thank You to atoman For This Useful Post:

I would like to just say thanks a lot for all your quick and helpful advice so far atoman, i appreciate your assistance!

I took a look at the wiring diagram for the IAT sensor, and from what i see, the 5V supply MUST pass through the TPS before it reaches the IAT... Wouldn't this drain the 5V a little in the first place?
In any case, after the TPS, there is a sensor ground and IAT branches in the circuit. Anyway i can confirm any resistance between TPS and IAT wires since they are in series?
Could the TPS be causing this problem?

UNLESS of course, the sensor signal from the PCM is what supplies the 5V.